The present invention relates to a technology for expanding the area in which an image is printed by a printer that uses interlaced printing to print images by forming dots while effecting sub-scanning.
In order to increase the printing speed of printers that use a dot printing head to form raster lines while performing sub-scanning at right-angles to the raster lines, most such printers are equipped with an array of nozzles arrayed in the sub-scanning direction. This is particularly true in the case of inkjet printers. In recent years, to achieve higher speeds, there has been a trend toward increasing the number of nozzles in the sub-scanning direction, increasing the size of nozzle arrays.
The interlaced printing system disclosed by U.S. Pat. No. 4,198,642 is one technology that is used to improve the quality of printing by such printers. FIG. 18 illustrates an example of interlaced printing. In this example, the number N of dot-formation nozzles is three. The pitch k of the raster lines laid down by the nozzles, that is, the spacing between nozzle raster line centers, is two raster lines. In the sub-scanning direction, the paper is fed in length L increments of three raster lines. In FIG. 18, the two-digit number in each circle denote the position at which each dot is printed. With further respect to the two-digit numbers, the digit on the left is the number of the nozzle used to print the dot, while the digit on the right indicates the number of the main scanning unit pass during which the dot is printed.
In the example of FIG. 18, during the first pass, dot raster lines are formed by the second and third nozzles, and the first nozzle is not used. The paper is then advanced by an amount corresponding to three raster lines, and the second main scanning unit is performed, during which a raster line is printed by each of the three nozzles. The image is printed by repeating this process of raster line formation and paper advance. The first nozzle is not used to form a raster line during the first pass to enable it to be used to form adjoining raster lines therebelow, during the second and subsequent passes.
Interlaced printing enables the effect of variations in nozzle pitch, sub-scan feed precision, ink emission properties and the like to be diffused, thereby enabling the image quality to be improved by reducing the deviation in dot position caused by such factors. Interlaced printing can be implemented using various paper feed amounts according to the number of scanning passes to complete dot formation on each raster line, the number of nozzles, nozzle pitch and so forth.
However, a problem with interlaced printing is that at the lower end of the paper there is an area in which printing cannot be accomplished. This is shown by FIG. 19, which illustrates an example of interlaced printing using a print head having seven nozzles disposed at a pitch corresponding to four raster lines. The paper is fed in the sub-scan direction in set increments of seven raster lines each. The circles represent the position of the head in the sub-scanning direction, during each main scanning unit pass. The numbers in the circles are the nozzle numbers.
FIG. 19 shows seven main scanning unit passes, from Lxe2x88x926 to L, at the lower end of the page being printed. In this explanation, nozzle number 7 in main scanning unit pass L is taken to be at the lower limit at which a nozzle can be positioned during printing. As can be seen, interlaced printing gives rise to a blank raster line, a raster line on which dots are not printed, shown here as the raster line adjoining the lower end of the area A. This means that printing cannot be effected in the 18-raster line area that extends from the area A down to the lower limit.
The greater the number of nozzles there are in the array, the larger the unprintable area becomes. The growing size of nozzle arrays in recent years has made it impossible to ignore this unprintable area. A large such area reduces the value of a printer. Moreover, with the demand being for higher print quality and faster speeds, reducing the size of the unprintable area at the expense of print quality is unacceptable.
This invention was accomplished to overcome the foregoing problems of the related art and has as its object to provide a technology which, during interlaced printing, ensures an adequate printable area without any excessive loss of print quality.
The present invention provides a printer having the following configuration.
In accordance with the present invention, there is provided a printer that prints an image on a print medium by using a print head to form raster lines in a primary direction and scan in a sub-scan direction at right-angles to the raster lines. The printer comprising:a print head having a plurality of dot formation elements disposed in a sub-scanning direction at a prescribed spacing that is not less than two dots, and dot formation elements for forming dots of a different type, disposed separately in the sub-scanning direction; a first transport unit that effects sub-scanning by transporting at a first precision at least one selected from the print head and the print medium;a second transport unit that, when sub-scanning cannot be effected by the first transport unit, effects sub-scanning by transporting at a second precision that is lower than the first precision at least one selected from the print head and the print medium;a data supply unit that assigns to the dot formation elements raster data for forming raster lines for printing the image, supplies the data to the print head, and also performs masking of dot formation elements where there are no raster lines to be formed;a raster line formation unit that drives the print head to form each raster line in accordance with the data supplied by the data supply unit; and a sub-scanning unit that performs sub-scanning by a preset feed amount when there arises a condition in which, in sequential usage of the first and second transport unit, an area is formed in which raster lines formed during sub-scanning effected by the second transport unit are included between raster lines formed during sub-scanning effected by the first transport unit. This is intended for printers with print heads having formation elements for different types of dots arrayed in the main scanning unit direction, such as a multicolor printer having a print head in which the elements for forming the dots of each color are arrayed in the main scanning unit direction.
Printers usually have a unit for effecting high-precision sub-scanning during printing, and a unit of ejecting the paper after the printing is finished. In the case of the printer of this invention, the first transport unit is used as the former and the second transport unit as the latter. Moreover, when sub-scanning by the first transport unit is no longer possible, printing continues using the second transport unit to effect sub-scanning. As explained above, when printing by the interlaced method, although sub-scanning may be effected down to the lower limit with respect to nozzle position, below that is an unprintable area. In this respect, the second transport unit of the printer of this invention enables the lower limit of sub-scanning to be extended, thereby increasing the area that can be printed using interlaced printing.
In addition, sub-scanning by the printer of this invention produces an area that includes raster lines formed by the second transport unit between raster lines formed by the first transport unit. The first and second transport unit are used sequentially, so printed images are comprised of an area printed by the first transport unit (hereinafter also referred to as xe2x80x9chigh-precision areaxe2x80x9d), an area comprised of raster lines formed by both the first and second transport unit (hereinafter also referred to as xe2x80x9cmixed-raster line areaxe2x80x9d), and an area printed by the second transport unit (hereinafter also referred to as xe2x80x9clow-precision areaxe2x80x9d), in that order, or the reverse thereof, in the direction of the sub-scanning.
Since the precision of the second transport unit is lower than that of the first transport unit, the image quality of the resultant low-precision area is not as good as that of the high-precision area. If a high-precision area and a low-precision area are positioned adjacently with no mixed-raster line area between them, the drop in image quality is more pronounced, and deviations in the dot formation positions can give rise to pseudo-outlines at boundaries. The printer of this invention makes the transition between high- and low-precision areas less noticeable by providing a mixed-raster line area between them, also enabling the suppression of pseudo-outlines at boundaries. Thus, the printer according to this invention enables the printable area to be increased without decreasing the quality of the printed image.
Although the foregoing explanation was made with reference to an arrangement in which the second transport unit is used to eject the paper, the invention is not limited thereto. Instead, the invention can be applied to various types of printer having two transport unit that differ in terms of transport precision and area, regardless of whether or not such unit are used to eject the paper.
The printer of the invention is capable of various feed settings that satisfy the above conditions. For example, the sub-scan feed amount effected by the second transport unit may be the same as that effected mainly for sub-scanning by the first transport unit.
The fact that interlaced printing gives rise to blank raster lines, as described with reference to FIG. 19, unit that there is a limit to the area that can be printed at high precision. However, as is clear from the example of FIG. 19, in the unprintable area, high-precision printing of raster lines is intermittently possible during main scanning unit passes Lxe2x88x922 to L. Blank raster lines, such as the ones shown in the drawing, can be formed one after the other by continue to effect sub-scanning using the second transport unit, making it possible to produce an area of raster lines formed by both the first and second transport unit.
In the example of FIG. 19 in which the nozzle pitch and number of nozzles are specified, the same type of printing can be effected with interlaced printing. That is, regardless of whether sub-scanning is effected by the first or second transport unit, a mixed-raster line area can be formed using a prescribed sub-scan feed amount common to both. In accordance with the inventive printer having the above configuration, it is not necessary to adjust the control of the sub-scanning according to the transport unit used, which makes it very easy to control to achieve printing that includes a mixed-raster line area.
In the case of FIG. 19, the sub-scan feed is in set increments each corresponding to seven raster lines. In the case of the present invention, the feed amount does not have to be fixed. Instead, depending on the nozzle pitch and the number of nozzles, interlaced printing may be performed using a set cycle of a plurality of different feed amounts, with each feed amount being applied without modification.
It is also not necessary to use the same feed amounts for high-precision, mixed-raster line and low-precision areas. Particularly in the case of low-precision areas, it is desirable to set a compensatory feed amount to make up for the lower precision.
For example, the sub-scan feed amount effected by the second transport unit could be set to be smaller than the average feed amount effected in the sub-scan direction by the first transport unit.
Decreasing the size of the sub-scan feed amount increases the precision of the sub-scanning. As such, in the case of a low-precision area, it is possible to compensate for the lower sub-scanning precision by using a sub-scan feed amount that is smaller than that used in a high-precision area. Even in a low-precision area, this makes it possible to maintain an image quality that is on a par with that of a high-precision area. xe2x80x9cAverage feed amountxe2x80x9d refers to the set feed amount by which sub-scanning is effected in a high-precision area; in cases in which sub-scanning is effected using a cycle of a plurality of different feed amounts, the average feed amount unit the average of the plurality of feed amounts. The average feed amount may also be used to evaluate the sub-scan feed effected by the second transport unit.
As another setting, the sub-scan feed by the second transport unit may be set at an amount that enables the formation of raster lines by a number of elements that is larger than the number of elements required to form raster lines during sub-scanning by the first transport unit.
With such a setting, overlap printing is used in low-precision areas. In overlap printing, each raster line is printed using a plurality of formation elements. Printing the dots of odd-numbered pixels during a first pass and using different elements to print the dots of even-numbered pixels during a second pass is an example of such a method. Like interlaced printing, overlap printing can improve print image quality by diffusing the effect of the positional deviation of dots in the image. In the case of a low-precision area, the above setting makes it possible to compensate for the lower sub-scanning precision by using overlap printing implemented using a larger number of elements than the number of elements used for a high-precision area. This unit that even in a low-precision area, it is possible to maintain an image quality that is on a par with that of a high-precision area.
The printer of the present invention can also be configured to perform printing using just a feed amount satisfying the conditions described in the foregoing. For this, it is desirable that the printer also includes a second sub-scanning unit that uses just the first transport unit to effect sub-scanning in preset feed amounts, and a selection unit that, based on specific conditions, selectively uses the sub-scanning unit and second sub-scanning unit.
In a first printing mode, the printer thus configured has a first printing mode in which the printing includes the printing of high-precision, mixed-raster line and low-precision areas, and a second printing mode in which only the high-precision area is printed. The selection unit enables the appropriate mode to be used. This enables the required printing to be achieved and makes the printer more convenient.
Various selection conditions can be set in the printer. For example, a selection command can be used to select between the printing mode using the above-described sub-scanning unit and the printing mode using the second sub-scanning unit, with the choice depending on the user""s requirements.
Although the first printing mode includes the printing of low-precision areas, it has the advantage of a larger printable area than that of the second printing mode. This feature can be used as a basis for automatically selecting the printing mode. For example, the system can be configured to select the sub-scanning unit when the image to be printed is above a certain size.
This makes the printer much more convenient to use. The two selection methods may be applied in combination so that when the image is larger than a certain size, the user can specify the appropriate mode. The basis of the mode selection is not limited to this, and may include other criteria such as whether text or graphic data is to be printed, the type and thickness of the paper, the overlapping method, resolution and other such printing conditions.
When printing is performed using the second sub-scanning unit, sub-scanning in at least an area located at one end in the sub-scanning direction can also be performed using a smaller average feed amount setting than the average feed amount used for sub-scanning in a central area.
With this configuration, the size of the sub-scan feed amount is reduced in the vicinity of the lower end of the high-precision area. By thus enabling raster lines to be formed more densely in the sub-scanning direction, the area that cannot be printed during interlaced printing can be decreased. In effect, the high-precision printing area is expanded.
This invention can be applied to various types of dot printer. Particularly in the case of an inkjet printer, it is desirable to situate the first transport unit upstream of the second transport unit in terms of the direction of sub-scanning.
Owing to variations in the ink emission characteristics, positional deviation of dots is a common occurrence in the case of inkjet printers. To prevent bleeding or color mixing between adjacent dots, it is preferable to leave some time between the formation of adjacent raster lines. In view of the fact that interlaced printing is particularly effective when used by inkjet printers, this makes the present invention highly applicable.
A page printed by an inkjet printer has to be protected until the ink is sufficiently dry. However, it is difficult to protect the printed surface and at the same time perform high-precision sub-scanning. Therefore, the printer of this invention can be configured with the first transport unit on the upstream side of the sub-scanning and the second transport unit on the downstream side.
The first transport unit can be constituted with friction rollers on the upstream side to transport the printing paper, while the second transport unit can be constituted to transport the paper on the downstream side with friction rollers having a lower frictional force than the rollers on the upstream side.
The invention also comprises a method of printing images on a print medium.
The present invention also comprises a recording medium recorded a computer program used to drive a printer to print images.
The printer described in the foregoing can be realized by the execution of the programs. The recording medium may be a floppy disk, CD-ROM, opto-magnetic disk, IC card, ROM cartridge, punched cards, printed material on which bar codes or other such symbols are printed, internal computer storage unit such as RAM or ROM memory, as well as external storage unit and other such computer-readable media. A configuration can also be used in which the computer programs for effecting the various functions are supplied to the computer via a telecommunication channel.
Each of the aspects of the invention described above can be applied to printers in which raster lines are formed while the print head is moved reciprocally in the main scanning unit direction, relative to the print medium, and to printers that are provided with multiple printing elements arrayed in the direction of the raster lines that enable the raster lines to be formed without main scanning unit.